1. Field of the Invention
This invention relates to a sheet feeding apparatus for feeding sheets, and an image forming apparatus and an image reading apparatus provided with the same, and particularly to such apparatuses provided with separating and feeding means adapted to separate and feed sheets one by one by the use of a retard separating method.
2. Related Background Art
An image forming apparatus or an image reading apparatus according to the prior art is provided with a sheet feeding apparatus for conveying recording sheets to an image forming portion or an original to an image reading portion. FIG. 6 of the accompanying drawings shows the construction of such a sheet feeding apparatus according to the prior art, and in FIG. 6, the reference numeral 1 designates a sheet feeding apparatus for supplying recording sheets 2 to an image forming portion, not shown.
This sheet feeding apparatus 1 is provided with a pickup roller 5 for feeding out the uppermost recording sheet 2a of recording sheets 2 stacked on a stacking plate, not shown, provided on a sheet containing device 3, a feed roller 6 for conveying the recording sheet 2a fed by the pickup roller 5 to an image forming portion, a retard roller 7 brought into pressure contact with the feed roller 6 and rotated in the same direction as or the opposite direction to the direction of rotation of the feed roller 6, and a pair of conveying rollers 9 provided downstream of the feed roller 6.
A first guide 10 is disposed between the pickup roller 5 and the combination of the feed roller 6 and the retard roller 7, a second guide 11 is disposed between the combination of the feed roller 6 and the retard roller 7 and the pair of conveying rollers 9, and further a third guide 12 is disposed downstream of the pair of conveying rollers 9, and the recording sheets 2 may be guided and conveyed by these guides 10 to 12.
Here, the retard roller 7 which is a separating rotary member cooperates with the feed roller 6 which is a feeding rotary member to constitute separating and feeding means for separating and feeding the recording sheets 2 fed out by the pickup roller 5 which is sheet feeding means one by one, and when a plurality of recording sheets 2 are fed from the sheet containing device 3, the retard roller 7 is adapted to be rotated in the opposite direction to the direction of rotation of the feed roller 6, as shown in FIG. 6, to thereby separate the uppermost recording sheet 2a from the other recording sheets 2.
On the other hand, FIG. 7 of the accompanying drawings shows the construction of a drive transmitting device for driving the feed roller 6 and the retard roller 7, and in FIG. 7, the reference numeral 13 denotes the drive transmitting device. This drive transmitting device 13 is provided with a feed roller shaft 15 on which the feed roller 6 is mounted, a retard roller shaft 16 on which the retard roller 7 is mounted, and a retard roller driving shaft 17 which is a driving shaft connected to the retard roller shaft 16. The retard roller shaft 16 and the retard roller driving shaft 17 are provided substantially in parallelism to the feed roller shaft 15 and supported by a pivotally movable supporting member, not shown, and movable toward and away from the feed roller shaft 15.
Also, the reference numeral 20 designates a torque limiter provided between the retard roller shaft 16 and the retard roller driving shaft 17, and the reference numeral 22 denotes an electromagnetic clutch provided on the end portion of the feed roller shaft 15 for transmitting a driving force transmitted from the main driving means of the main body of an image forming apparatus, not shown, through a driving input belt 21 to the feed roller shaft 15.
Also, the reference numeral 23 designates a retard driving belt extended between the feed roller shaft 15 and the retard roller driving shaft 17 for transmitting a rotatively driving force transmitted to the feed roller shaft 15 further to the retard roller driving shaft 17. The reference numeral 19 denotes a coupling provided between the retard roller shaft 16 and the retard roller driving shaft 17 for transmitting the drive of the retard roller driving shaft 17 to the retard roller shaft 16.
Description will now be made of the driving of the feed roller 6 and retard roller 7 by the drive transmitting device 13 constructed as described above.
When a rotatively driving force is given from the main driving means, this rotatively driving force is inputted through the driving input belt 21 a pulley 25 provided on the armature portion, not shown, of the electromagnetic clutch 22 ON-OFF-controlled in conformity with sheet feed timing. Here, the feed roller shaft 15 is adapted to rotate with the rotor portion, not shown, of the electromagnetic clutch 22, and the feed roller shaft 15 and the retard roller driving shaft 17 and the retard roller shaft 16 are connected together by the retard driving belt 23.
Thus, when the feed roller 15 is rotated, the retard roller driving shaft 17 is rotated in the same direction as the feed roller shaft 15, and further, when the feed roller 15 and the retard roller driving shaft 17 are thus rotated, the feed roller 6 and the retard roller 7 are rotatively driven in synchronism with the ON of the sheet feed timing.
Now, when the recording sheets 2 are fed one by one in the direction of arrow b in FIG. 6, the frictional force between the feed roller 6 and the recording sheet 2 acting on the retard roller 7 is greater than the returning force of the torque limiter 20 and therefore, the retard roller 7 and the retard roller shaft 16 are adapted to be driven to be rotated in the opposite direction to the rotatively driving direction of the retard roller driving shaft 17 while idly rotating the torque limiter 20.
Also, when a plurality of recording sheets 2 are fed, the frictional force among the plurality of recording sheets 2 is small relative to the frictional force between the retard roller 7 and the recording sheet 2 and therefore, the torque limiter 20 is not idly rotated, whereby the retard roller 7 and the retard roller shaft 16 are rotated in the same direction as the rotatively driving direction of the retard roller driving shaft 17.
By the retard roller 7 being rotated in such a direction, of the plurality of recording sheets 2 fed, the recording sheet 2a which is most adjacent to the feed roller 6, that is, which is uppermost, comes to be separated from the other recording sheets 2, whereby the plurality of recording sheets 2 can be prevented from being conveyed to the image forming portion. Hereinafter, the phenomenon that a plurality of recording sheets 2 are fed from the sheet containing device 3 to the retard roller 7 will be referred to as xe2x80x9cbundle conveyancexe2x80x9d, and the phenomenon that the plurality of recording sheets 2 cannot be separated from one another and are fed will be referred to as xe2x80x9cdouble feedingxe2x80x9d.
Description will now be made of theoretical expressions satisfying the feeding and separating conditions for the recording sheet 2 by such a sheet feeding apparatus 1.
Here, when the coefficient of friction between the feed roller 6 and the recording sheet 2 is defined as xcexcBP and the coefficient of friction between the retard roller 7 and the recording sheet 2 is defined as xcexcCP and the coefficient of friction between the recording sheets is defined as xcexcPP and the pressurizing force (pressure contact force) of the retard roller 7 is defined as N and the idling torque of the torque limiter 20 is defined as T and the effective radius of the retard roller 7 is defined as r and the returning force of the torque limiter 20 is defined as T/r, the feeding condition of the feed roller 6, the separating condition for the recording sheet 2 and the driven to be rotated condition of the retard roller 7 are expressed as follows:
Also, when T/rxc2x7N is defined as xcexcT, the foregoing feeding condition, separating condition and driven to be rotated condition are expressed as follows:
Now, FIG. 8 of the accompanying drawings is a graph showing the relations among the foregoing expressions (4), (5) and (6) with the pressurizing force N of the retard roller 7 and the idling torque T of the torque limiter 20 as parameters, and in FIG. 8, the hatched portion is a feeding area.
Here, to enlarge the hatched area, it is necessary to make the coefficients of friction between the respective rollers 6, 7 and the recording sheet great, and it can be understood from this figure that the feeding area will become wider if the feeding condition is set under a condition in a direction (rightwardly upward direction) in which both of the pressurizing force N of the retard roller 7 and the idling torque T of the torque limiter 20 are made great.
However, it is often the case that among recording sheets 2 usually used, the coefficient of friction between the sheets is irregular even if the sheets are of the same kind and depending on the user, recording sheets of different kinds are irregularly stacked on the stacking plate of the sheet containing device 3.
In such a case, if as shown, for example, in FIG. 9 of the accompanying drawings, the coefficient of friction between the lowermost recording sheet 2b of a bundle of recording sheets of a different kind and the recording sheet 2c just beneath it is remarkably small as compared with the coefficient of friction between the recording sheets near it, bundle conveyance occurs and the bundle-conveyed recording sheets 2 are conveyed to the nip portion N between the feed roller 6 and the retard roller 7, whereafter they are conveyed to the image forming portion while being separated. In FIG. 9, three recording sheets 2 are being bundle-conveyed.
However, when the recording sheets 2 are thus bundle-conveyed, even if the setting of the aforementioned feeding conditions (1) and (4) is changed, sometimes the bundle-conveyed recording sheets 2 cannot be separated and double feeding may occur.
This phenomenon will now be described with reference to a typical view of a bundle conveyance time shown in FIG. 10 of the accompanying drawings (FIG. 10 shows the conveyance of a bundle of three sheets).
Here, if the coefficients of friction xcexcPP among the recording sheets 2 are all equal or the coefficient of friction xcexcBP1 between recording sheet No. 1 and recording sheet No. 2 is xcexcBP1 less than xcexcBP2 relative to the coefficient of friction between recording sheet No. 2 and recording sheet No. 3, the bundle-conveyed recording sheets will be separated in the order of recording sheet No. 3 and recording sheet No. 2 by the retard roller 7 and double feeding will not occur.
However, when the state that xcexcBP1 greater than xcexcBP2 occurs, during the time until recording sheet No. 3 is separated by the retard roller 7 and passes through the nip portion N between the retard roller 7 and the feed roller 6, recording sheet No. 2 is dragged by recording sheet No. 1 and is fed by a predetermined distance toward the image forming portion. When recording sheet No. 2 cuts in between the pair of conveying rollers 9 before recording sheet No. 3 is completely separated, double feeding occurs as the result.
The behavior of the recording sheet (in FIG. 10, recording sheet No. 2) between the thus bundle-conveyed uppermost and lowermost recording sheets is determined by the coefficient of friction between the bundle-conveyed recording sheets 2 however parameters which are to determine the feeding condition, such as the pressurizing force of the retard roller 7, idling torque of the torque limiter 20 and the coefficient of friction between each roller and the recording sheet, may be varied.
The typical view of the bundle conveyance shown in FIG. 10 shows the conveyance of three sheets, but under the situation in which as in recent years, the kind of recording sheets is diversified the bundle conveyance of fine to ten sheets is not rare and of course, it makes the situation of separation more severe that the number of bundle-conveyed sheets increases.
Heretofore, to improve the separating performance in the feeding condition at any rate, for example, the idling torque of the torque limiter 20 has been made great and the pressurizing force of the retard roller 7 has been made small.
However, such a feeding condition is a feeding condition under which the wear of the retard roller 7 by the recording sheets 2 is liable to occur and therefore, the durability of the retard roller 7 is remarkably reduced. Further, the already described bundle conveyance occurs more remarkably in a high-speed apparatus wherein the durability of parts is required, and the setting of such a feeding condition that the durability of the retard roller 7 is reduced becomes a countermeasure adverse to the directionality of product.
So, heretofore, the feed roller 6 has been formed of a rubber material such as EPDM, while the retard roller 7 has been of such structure as shown in FIG. 11 of the accompanying drawings, that is, structure in which a sponge base material 151 is provided on the surface of a core material 150 and further, a silicon coat layer 153 is secured to the surface of the sponge base material 151 by a urethane adhesive layer 152.
The reason why the silicon material is used as the outermost layer of the retard roller 7 is that it is good for the continuation of a high coefficient of friction xcexc, and it is for the purpose of preventing the pluck, planing or the like of the surface of the sponge and improving the durability of the retard roller 7 that the urethane adhesive layer 152 is provided as the intermediate layer.
When as described above, a sponge roller having the silicon coat layer 153 is used as the retard roller 7, the hardness of the retard roller 7 is smaller than the hardness of the feed roller 6 and therefore, when the two rollers 6 and 7 are in pressure contact with each other, the shape of the nip therebetween becomes a shape depressed toward the retard roller 7 side.
Therefore, when a bundle of recording sheets are fed into this nip portion by the pickup roller 5, the leading ends of the recording sheets get out of position in the form of roofing tiles, whereby the leading end portions of all the fed recording sheets become capable of directly contacting with the retard roller 7. As the result, even various recording sheets differing in thickness and coefficient of friction from one another can be reliably separated and fed by the retard roller 7 and thus, an improvement in the stability of feeding of the recording sheets can be achieved.
However, when in a conventional sheet feeding apparatus provided with such a retard roller, in order to enhance the separating performance as already described, the idling torque of the torque limiter 20 is made great and the pressurizing force of the retard roller 7 is made small, the durability of the retard roller 7 is remarkably reduced.
The conveying incapable state said to be the so-called durable life of the roller will now be described on the basis of the situation of change in xcexc (coefficient of friction) on the feed roller 6 and retard roller 7 during the endurance thereof.
As the endurance of the feed roller 6 and retard roller 7 progresses, the aforementioned xcexcBP (the coefficient of friction between the feed roller 6 and the recording sheet 2) and xcexcCP (the coefficient of friction between the retard roller 7 and the recording sheet 2) change following the process as shown in FIG. 12 of the accompanying drawings. When as already described, use is made of the retard roller 7 formed of sponge and having the silicon coat layer 153 on the outermost layer, the initial xcexcBP and xcexcCP are both high and stable at 2 or greater, and the situation of reduction in xcexc during the endurance of the two rollers substantially likewise progresses at xcexcBPxe2x88x9dxcexcCP.
When the endurance of the feed roller 6 and retard roller 7 further progresses, the aforementioned expression (3) soon comes to break (that is, expression (1) also breaks) and comes to be below a straight line indicated by xcexcT (=T/rxc2x7N) in FIG. 12. This straight line is a line indicative of the boundary where the retard roller is driven to be rotated, that is, when the coefficient of friction xcexcCP between the retard roller and the recording sheet 2 is above this straight line, the retard roller is driven to be rotated by the recording sheet 2 being conveyed, and when xcexcCP is below this straight line, slip occurs between the retard roller and the recording sheet 2 and the retard roller 7 slips.
When as described above, xcexcBP and xcexcCP are below the straight line indicated by xcexcT (=T/rxc2x7N), the retard roller 7 has come not to be driven to be rotated by the recording sheet 2 being conveyed, or the feed roller 6 has become incapable of conveying the recording sheet 2 and has thus met the end of its life.
Besides, when for example, particular paper containing a great deal of calcium carbonate therein is to be conveyed, both of xcexcBP and xcexcCP are severely reduced for a small number of passed sheets under the influence of paper powder produced during the conveyance, and come to be below xcexcT, and the feed roller has sometimes come to meet the end of its life within a short period.
So, the present invention has been made in view of such circumstances and has as its object to provide a sheet feeding apparatus which can achieve an improvement in the durability of separating and feeding means, and an image forming apparatus provided with the same.
The present invention provides a sheet feeding apparatus for separating and feeding sheets contained in sheet containing means one by one, the sheet feeding apparatus being provided with:
separating and feeding means for separating and feeding the sheets one by one, the separating and feeding means being provided with a feeding rotary member rotatable in a direction to feed the sheets, and a separating rotary member brought into pressure contact with the feeding rotary member and rotatable in a direction to return the sheets t the sheet containing means; and
a torque limiter provided between the separating rotary member and a driving shaft for driving the separating rotary member, for transmitting the drive from the driving shaft to the separating rotary member, and idly rotated when predetermined rotational torque in a reverse direction is transmitted to the separating rotary member;
wherein the coefficient of friction of the separating rotary member is set so that the relatives that
xcexcBC N greater than T/r
xcexcBP greater than xcexcCP
T/r greater than xcexcCP N greater than xcexcPP N
may be established among the coefficient of friction xcexcBC between the feeding rotary member and the separating rotary member, the coefficient of friction xcexcBP between the feeding rotary member and the sheet, the coefficient of friction xcexcCP between the separating rotary member and the sheet, the coefficient of friction xcexcPP between the sheets, the pressure contact force N of the separating rotary member, the idling torque T of the torque limiter and the effective radius r of the separating rotary member.